Optical scanners of the prior art are provided with a line detector, e.g. in the form of a CCD line array in which the information from the original is collected and processed electronically as pixels. A CCD line array may have e.g. a plurality of light sensitive cells corresponding to 5000 pixels. Several CCD line arrays may be combined to form larger units. The optics of the scanner thus decides the real pixel resolution of the scanner. The pixel resolution may be e.g. 800 dpi. Variations in the pixel resolution of the scanner may be obtained by performing digital averaging on the read image information. Alternatively, the video signal from the line array of the scanner may be resampled with the desired clock frequency. However, it is important that these processing steps can take place directly when data are read, since this obviates the necessity of intermediate storing of data before the resolution of the image is varied.
It is often desired to vary the resolution of the scanner, because data are subsequently to be processed on a PC or a corresponding work station, and it is therefore important that the scanner can supply data which can be used directly in the subsequent data processing.
While data along the read lines can be processed by averaging and the like, the feed rate of the original has so far been used for varying the line resolution of the scanner. This means that the scanner feed rate for the original is increased if the resolution is to be reduced. The line resolution of the scanner might be halved simply by throwing away every second line, but this solution is not acceptable if the desired line resolution cannot be obtained by dividing the desired basic resolution. That is if the desired line resolution is different from 800 dpi, 400 dpi, 200 dpi, etc.
In connection with colour scanners three colour signals are usually picked up from the original, and since the line sensors for the individual colours are offset from each other in the image plane of the lens, the individual sensors pick up information from lines which are offset from each other in the object plane of the lens. Since it is simultaneously desired that the detector is to collect information for the individual colours in one and the same line, it is not possible to vary the feed rate of the original to obtain the desired variable line resolution.
To ensure a variable resolution, there is described a resolution in which a line on an original is imaged as three lines on the line detector. This is achieved by using a colour separator with dichroic coatings. This is described e.g. in Hewlett-Packard Journal, August 1993 by K. Douglas Gennetten et al.